The oxidative cleavage of phenols, catechols and orthobenzoquinones with a reagent prepared from cuprous chloride, oxygen, pyridine and methanol is known. The product is the monomethyl ester of cis,cis muconic acid. The dimethyl ester of cis,cis muconic acid has also been prepared. It has been disclosed that monoesters can also be prepared by reacting catechols and 4-tert-butyl-1,2-benzoquinone with a copper (II) reagent generated from oxygen, cuprous chloride in pyridine and any of the following alcohols: methanol, ethanol, n-butyl alcohol and isopropyl alcohol.